For this purpose, tear film thickness will be measured at baseline and after single instillation of one of 5 study drugs in one randomly chosen eye. In addition, one group of 20 subjects will receive no drug and will serve as a second control. Drug effects on tear film thickness will be compared to the fellow, non-treated eye. In addition, effects on tear film thickness of timolol with preservatives (Timoptic 0.5%) will be compared to timolol without preservatives (Timophtal sine 0.5%) and three lubricants with different viscosity (Genteal HA, Hylo-Comod, Thealoz).